Frequency division multiplexed communications systems transmit many, sometimes upwards of thousands, of carrier signals, simultaneously to communicate information. Transmitted carrier signals are sometimes referred to as tones. In the case of OFDM systems the transmitted tones are orthogonal to each other to thereby avoid or minimize mutual interference. In an OFDM system, each tone may be used to transmit a different unit of data, e.g., symbol, in parallel.
For each transmitted carrier signal, an OFDM receiver normally attempts to compensate for the distortion induced by the transmission channel. This will normally involve a channel estimation operation and a channel compensation operation. To assist a receiver in overcoming multipath distortion, pilot signals with known data patterns are transmitted. The pilot signals, sometimes called pilot tones or simply pilots, are used to support channel estimation operations. Such channel estimation operations normally attempt to estimate the amplitude and phase distortion introduced by the communications channel.
The pattern structure of the pilots can be in essentially any manner, provided that the Nyquist sampling criteria for the communication channel's impulse response and rate of change are satisfied. The number of pilots transmitted is often a function of the expected multipath distortion delay and the anticipated rate of change in channel conditions. However, for purposes of efficiency, it is desirable to minimize the number of pilots transmitted since the transmission of a pilot precludes the transmission of data in the transmission slot used to transmit the pilot.
FIG. 1 is a chart 100 which provides a simple example of a pilot pattern that is typical of an OFDM signal. In FIG. 1, the horizontal axis corresponds to frequency with each rectangle in a row corresponding to a different tone of an OFDM signal. While twenty-five tones are shown, any number, e.g., thousands, of tones may be transmitted in parallel as part of an OFDM signal during a single symbol time period. In the FIG. 1 example, the vertical axis corresponds to time as measured in terms of symbol time periods. One symbol, e.g., a QAM symbol, is transmitted per symbol time period using each tone.
Pilots 102 are represented in FIG. 1 as dark dots. They are distributed in frequency and time in the illustrated grid representation. In the FIG. 1 example, pilots are transmitted during every other symbol transmission period. Accordingly, there are symbol transmission periods during which no pilots are transmitted or received. In such an implementation, during some symbol periods, no pilot tones are received.
The received known pilot signals are used to estimate the channel distortion at the time and frequency of each pilot. For each OFDM tone a channel estimate is normally required for channel compensation purposes. Thus, where no pilots are located, e.g., for each frequency/time slot used to transmit data as opposed to a pilot tone, a channel estimate needs to be generated. Pilots for these frequency/time slots are normally generated using interpolation on the received pilots in the time and/or frequency domain. In such systems two dimensional, or two independent interpolations, across frequency and time may be performed to fill in the missing pilots used to provide channel information. An exemplary pilot interpolation process is shown in FIG. 2, with interpolation being performed across the frequency domain using pilots represented by arrows 208 received during the same symbol time period.
The pilot interpolation can be performed using a number of known techniques. One can perform a simple linear interpolation between pilot data points or more sophisticated cubic interpolation. Perhaps the most popular approach to “filling in the gaps” between pilot “bins” is to perform a low pass filtering (LPF) operation on the received pilot data points. In the context of the present application a “bin” represents data or a set of data corresponding to an individual carrier frequency (tone). Accordingly, when processing an OFDM signal having M tones, M bins will normally be used. The LPF interpolation process not only fills in the data between the pilot bins, but also allows for noise reduction due to the LPF operation. FIG. 2 illustrates an example of the result of low pass filtering to interpolate between pilot data points to generate a complete set of channel estimation information for one symbol time period. The FIG. 2 illustration may be the result of, e.g., performing interpolation on the pilots of the first row of data shown in FIG. 1. In the FIG. 2 example, each dot corresponds to an interpolated value at a carrier frequency. In the FIG. 2 example valleys such as the valley 204 correspond to carriers where there is low channel noise while the peaks 202, 206 correspond to carriers where the channel has relatively high noise.
Unfortunately even with pilots spaced close enough in frequency and time to meet the channel's Nyquist criterion, filling in the channel estimate between the known pilot bins is still prone to error due to additive noise. Known channel estimation techniques attempt to solve this problem in the following manner. The OFDM symbols are received, the pilots are extracted, then averaged over many OFDM symbols. Once sufficient averaging in performed providing a reduction in noise corruption, the channel is interpolated between the pilots. The received signal on a given channel is then multiplied by the inverse of the corresponding channel estimate in an attempt to remove multipath and/or other distortion introduced by the communications channel.
This known technique works well except that it depends on a relatively long integration time to reduce the noise corruption. This delay, due to the need to average multiple pilot tones before a reliable channel estimate is generated, is undesirable since it increases the time between when a carrier recovery signal lock is first achieved and when received symbols may be decoded in a reliable manner.
In view of the above discussion, it is apparent that there is a need for methods and apparatus which can be used to reduce the amount of time required to produce reliable channel estimates in frequency division multiplexed systems. From a transmission efficiency standpoint, it is desirable that at least some of the methods and apparatus be capable of reducing the amount of time required to produce reliable channel estimates without requiring the transmission of more pilot tones then are transmitted in the known systems.